Hitherto, in a heat pump apparatus such as an air-conditioning apparatus or a water heater, a vapor-compression refrigeration cycle using a refrigerant compressor is generally employed. That is, the vapor-compressor refrigeration cycle constructed by connecting the refrigerant compressor, a condenser, a pressure reducing unit, and an evaporator to each other through pipes is installed in the heat pump apparatus, thereby being capable of carrying out operation in accordance with intended use (such as air-conditioning use and hot water supply use).
Incidentally, in recent years, regulations on greenhouse gas emission were incorporated in the Kyoto Protocol in 1997 from the viewpoint of preventing global warming, and the Kyoto Protocol came into force as international law in 2005. In order to reduce the amount of carbon dioxide emission and achieve energy saving, in the field of HVAC, the heat pump apparatus has widely been spread in place of conventional water heaters and conventional room heaters, and efficiency of the heat pump apparatus has further been increased.
Regulations on energy saving of air-conditioning apparatus have been tightened in each country. In particular, as compared to the conventional standards, the latest standards have such a feature that energy-saving performance is evaluated under an operation condition closer to an actual load. Conventionally, in Japan, when indicating energy-saving performance, efficiency evaluation was indicated by a cooling-heating average coefficient of performance (COP) under a rated condition. Since 2011, the efficiency evaluation has been indicated by an annual performance factor (APF) calculated based on COPs under four cooling-heating conditions including a medium condition. In addition, in Europe, since 2012, there has been adopted a method of indicating energy-saving performance based on new standards for evaluating and calculating a seasonal energy efficiency ratio (SEER) of cooling and a seasonal coefficient of performance (SCOP) of heating under four cooling conditions and four heating conditions added to a low-load condition.
In this case, the low-load condition refers to a condition that a temperature difference between an outside temperature and an indoor temperature is small and a small quantity of heat is necessary for keeping the indoor temperature constant. The low-load condition also refers to a state in which a difference between a high pressure (Pd) and a low pressure (Ps) is small in the vapor-compressor refrigeration cycle and a small quantity of heat is necessary in a steady state (for example, 25% or less of rated capacity). Except for start of operation, about 10% to 50% of capacity under a rated condition is needed during steady operation, and a time period of operation in a range of from the low-load condition to the medium condition is longer than that of the rated operation. Accordingly, when substantially evaluating annual energy-saving performance, it is a new challenge to increase a COP in the low-load condition that is out of evaluation in the conventional standards.
Further, hitherto, ON-OFF control has been employed as a measure to adjust cooling or heating capacity, but has a problem in that a fluctuation range of temperature adjustment is wide and a vibration noise level is high, and another problem in that an energy loss is significant. In order to solve the problems, inverter control of varying a rotation speed of a drive motor of a compressor has widely been spread.
In recent years, the air-conditioning apparatus has been required to reduce a startup time period and to have increased heating capacity under a low-outside-temperature environment, and has been needed to have rated capacity with a certain level or more. On the other hand, houses are being highly air-tightened and heat-insulated, thereby reducing capacity necessary during the steady operation, and enlarging an operation capacity range. Accordingly, the air-conditioning apparatus is required to maintain high efficiency in a wider operation range and a wider rotation speed range, thereby being difficult to maintain high efficiency in operation performed at low speed and low-load capacity simply by employing control of the rotation speed performed by the related-art inverter.
In this context, a refrigerant compressor taking a measure to mechanically vary a displacement volume (mechanical volume control) comes into focus again. For example, in Patent Literature 1, there is disclosed such a configuration that in a rotary compressor of a two-cylinder rolling piston type, one of compression units is brought into a non-compression state under a low load, thereby halving a flow rate of circulation of refrigerant. In this configuration, operation can be carried out without reducing a rotation speed of an electric motor. Thus, efficiency of the compressor can be increased. As a specific measure, there is disclosed such a measure that when one of the compression units is brought into the non-compression state, a high pressure is led into one of cylinder chambers, and a pressure in a back pressure chamber on a blade (vane) back surface is changed to a medium pressure so that the blade (vane) is separated away from a rolling piston due to a pressure difference between the high pressure and the medium pressure, thereby bringing one of the compression units into the non-compression state (rest cylinder operating method).
Further, in Patent Literature 2, there is disclosed a configuration of a refrigeration cycle apparatus including a switching unit and a control unit for switching and controlling operation between single operation in which one of two compression units is brought into a non-compression state, and parallel operation in which both the two compression units are brought into a compression state in the two-cylinder (synonymous with a two-cylinder type) rotary compressor as disclosed in Patent Literature 1. The refrigeration cycle apparatus has a feature of including the control unit for controlling, based on an output frequency of an inverter circuit, switching performed by the switching unit, and for varying a point of the switching based on a temperature of a condenser of a refrigeration cycle.
Further, in Patent Literature 3, there is disclosed a multi-cylinder rotary compressor that accommodates an electric element in a closed shell having a high-pressure interior, and also accommodates therein a plurality of compression units to be driven by the electric element. As disclosed in Patent Literature 3, a tension spring for pulling the vane outward is arranged on a back surface side of a vane of at least one of the plurality of compression units, and another spring for pressing the vane inward is arranged on a back surface side (rear end portion side) of a vane of another one of the compression units. At the time of startup, a pressure difference between a discharge pressure and a suction pressure is small. Thus, on the compression unit side including the tension spring arranged on the vane, a pulling force of the tension spring is larger than a force for pressing the vane inward. That is, the pulling force of the tension spring overcomes the force for pressing the vane inward, and the vane is separated away from a rolling piston and brought into a non-compression state. This technology aims to secure a long period of time for light-load operation at the time of startup, to thereby perform startup gently and smoothly.